1. Field of the Invention
The present invention relates to an apparatus for measuring the gradient of a surface continuously with high accuracy.
In the manufacture of a phosphor screen of a color television picture tube, a slurry of red phosphors is coated on an inner surface of the face plate of the picture tube. A glass lens simulating the actual electron lens of the picture tube is mounted in front of the inner face plate of the picture tube and light is then projected through the lens where it is caused to strike on the regions of the phosphor and a color selecting grid (apertured grill or shadow mask) where it is caused to strike on the regions of the phosphor coating where the red phosphors should be. Light causes the slurry of red phosphors to be hardened, and the remaining slurry is then washed away. The process is then repeated by applying a green slurry to the inner surface of the face plate. The areas where the green phosphors are to appear in final form is then hardened by projecting a light through another lens and the remaining portions of the green slurry which have not been hardened are washed away. The process is then repeated for a third time for a blue phosphor. This is all diagrammatically shown in FIG. 1 of the drawings, where slurries of phosphors 2 are applied to the inner face of the face plate of the picture tube. 5 represents the lens, and 4 is the light source. The surface 5a of the lens 5 is curved so that light will fall on the phosphor regions of the phosphors after passing through a grid electrode or shadow mask 3. It is very important that the curvature of the face of the lens in front of the grid electrode or shadow mask have the gradients of this surface maintained within very close limits. In selecting a lens 5 for this operation, it is important that the surface be examined to determine its relative quality.
It will thus be understood that it is important that to correct the error caused by the approximation by light of the electron beam trajectory be accurately maintained.
2. Description of the Prior Art
In the prior art, it has been very difficult to measure the gradient of an arbitrary point on the curved surface, and it is impossible to control the quality of the corrective lens sufficiently to provide a proper quality of the picture screen.
In the prior art, there are disclosed several techniques to measure the physical dimensions directly or indirectly related to the gradient of a curved surface at many different selected points thereon. One method has been to measure the thickness of the lens mechanically by using a dial gauge. In this case, to convert the measured value of thickness into a gradient value, it is necessary to employ a very high density and precise sampling, which was not easy for such mechanical measurements. Furthermore, in this method, a needle directly contacts the curved surface of the lens, and thus the antireflective coating on the surface of the lens may be damaged by the needle.
Another optical measure of the angle has been disclosed in the prior art by using an auto collimator, but in this process, it was found difficult to obtain an image of the surface with sufficient light because it is required to restrict the field of vision to a small area, to measure the curved surface in which the gradient changes continuously. This was particularly true where the corrective lens is used in optical printing of a phosphor screen of a color television picture tube, since the gradient value is distributed in the range of a factor of .+-.10 milliradians. Commercially available auto collimators were not usable because the angles of the coverage of the measurement are different.
A projection method for using a collimated laser beam is also known in the prior art for the quality examination of the lens. In this case, the lens is covered by a mask except at particular points being examined through which the laser beam passes to make images on a photographic emulsion. But in this case, it is necessary to provide a mask, and it is impossible to examine each surface individually because light passing through the lens is used.